1. Field of the Invention
The present invention relates generally to a test device and, more particularly, to a device for testing the stators of dynamoelectric machines.
2. Description of the Prior Art
As is well known in the art, the stator core of a typical dynamoelectric machine has a generally cylindrical configuration and an annular transverse cross section. The stator core includes a longitudinally extending bore through it's center and is formed from a plurality of electrical grade steel laminations. Upon assembly, these laminations form a plurality of stator teeth extending circumferentially around the stator. The plurality of teeth are arranged to define a plurality of channels, generally referred to in the art as stator slots, which extend longitudinally over the length of the stator between adjacent teeth and are arranged to receive electrical conductors. Each lamination is coated with a thin layer of electrically insulating material which prevents the 60 hertz alternating magnetic flux generated in the stator core during machine operation from inducing eddy currents between adjacent laminations- In most dynamoelectric machines, the laminations are electrically connected together at their respective radial end portions where they are supported by the stator frame.
The dynamoelectric machine further includes a cylindrical rotor which is disposed within the stator bore and extends substantially the longitudinal length of the stator. A plurality of electrical windings are disposed on the rotor for inducing a voltage on the stator windings as the rotor rotates.
As is well known in the art, if the electrically insulating material of a particular lamination is defective at a location near the stator bore, a current conducting path is formed through the lamination to the stator frame. Current is induced to flow through the conducting path by the alternating flux generated as the rotor turns. This current flow causes localized lamination heating typically referred to as a "hot spot".
Hot spots can also occur in dynamoelectric machines even though the plurality of laminations are effectively electrically insulated from the stator frame. In this case, the current conducting path is typically completed through a particular lamination and through damaged electrical insulating material between adjacent laminations. For example, the electrical insulating material between adjacent laminations (interlaminar insulation) may be damaged during assembly or maintenance of a stator, particularly during the removal and replacement of the rotor. Hot spots may also be caused by foreign objects or by general deterioration of the interlaminar insulation. If hot spots are undetected, they can increase in magnitude until they cause one or more laminations to melt and may even damage the electrical insulating material surrounding the electrical conductors positioned in the stator slots.
Presently, test devices are available for detecting hot spots in stator cores. For example, stator cores may be tested for hot spot-induced damage using a "thermovision" test. In this test the stator core is excited by a winding to its full rated flux. Any hot spots on the stator teeth are readily detected with an infra-red camera which scans the stator bore. However, this test is unlikely to detect deep-seated hot spots unless more sophisticated temperature measurements are made.
U.S. Pat. No. 4,803,563, which is assigned to the assignee of the present invention, discloses a carriage for carrying a stator test device. When the rotor is disposed within the stator, the annular space, referred to in the art as an air gap, between the rotor and stator is minimal. This patent discloses a carriage that is slidably movable within this annular space between the rotor and stator and has a construction for receiving a testing device for stator inspection. The disclosed carriage allows testing of the stator without costly and time consuming removal of the rotor.
Another type of apparatus suitable for testing stator cores to detect hot spots is a commercially available apparatus referred to as an Electromagnetic Core Imperfection Detector, commonly referred to in the art as an ELCID. The ELCID includes a rectangular box-shaped housing or guide for receiving a chatrock coil. The housing has an open bottom face, and a rectangular, plate-shaped base is attached over a portion of the bottom face for mounting the ELCID to the stator. The longitudinal length of the base is disposed perpendicular to the longitudinal length of the bottom face forming a substantially T-shaped configuration. The base of the ELCID includes two movable, generally rectangular block type structures disposed parallel to each other and arranged to fit within a stator slot. These block structures are adjustable to fit within slots of various widths. To use the ELCID, these two block-type structures are matingly disposed between adjacent stator teeth in a single stator slot. The ELCID is moved manually along the length of the stator and the block structures function as a guides to maintain the ELCID along a straight path defined by the stator slot. The block structures of the ELCID are then placed in adjacent stator slots, and testing is repeated. Using this method, the region tested by the ELCID is positioned between the opposing outer edges of adjacent stator teeth. Testing in this manner ensures that the entire stator is tested because the tested region is well defined, and the possibility of inadvertently failing to test a stator section is avoided.
The previously mentioned chattock coil has two terminations disposed on the lower portion of the housing beside the base. These terminations each have a protruding lip portion functioning as a secondary guide for the block structures. When the ELCID is operational, each lip portion is disposed on a stator tooth corner and forms a seat for the stator tooth corner.
The ELCID requires that the stator core be excited to only about 3 to 5 percent of its full rated flux, which is sufficient to induce eddy currents to flow in a damaged insulation region of the stator. Because the current is very small, however, the damage-induced heating is insignificant. As a result, the ELCID test relies on electromagnetic detection of the axial fault currents flowing through the damaged region.
The chattock coil is used to measure the magnetic scaler potential between adjacent teeth to detect the presence of fault currents. The output of the coil is detected and amplified to yield a D.C. voltage proportional to the component of fault current. The chattock coil signal is compared in a signal processor to a constant signal derived from a reference coil which is maintained at one position in the stator. The purpose of the reference signal is to provide phase information required to make the processor more sensitive to axially flowing currents and thereby increase discrimination with respect to the fault currents.
Under certain conditions the ELCID may either be very difficult to operate or completely inoperable. For example, the stator slot depth may be minimal due either to deterioration or by design. This condition renders the block structures of the ELCID inoperable to fit within the stator slots. Further, the stator teeth corners may be rounded due to deterioration or by design, preventing the lip portions of the ELCID from seating on the stator teeth corners. Under these circumstances, neither the block-type structures nor the lip portions forming secondary guides are operable to maintain the ELCID in a straight path as it is moved along the stator. If the guides of the ELCID are not operable, the ELCID moves in an non-linear path when it is pushed along the stator by test personnel. This results in undesirable test results since portions of the stator are not tested. Portions of the stator may go untested since a non-linear testing path causes the boundary of a given testing region to waiver, making it difficult to determine exactly what region was previously tested. If the boundaries of adjacent testing paths are not contiguous, portions of the stator may go untested. To compensate for certain portions possibly being untested, retesting may be done to ensure that all regions of the stator are tested. Obviously, this increases the time and costs of testing the dynamoelectric machine.
Therefore, what is needed is an improved test device for testing a stator core which is operable to travel in a straight path defined by the stator slots as the test device is manually moved longitudinally along the stator.